(1) Field of the Invention
This invention relates to an apparatus for use with a spin coating apparatus for dispensing a coating solution to form silica film on surfaces of substrates such as semiconductor wafers or glass substrates for liquid crystal displays.
(2) Description of the Related Art
A conventional apparatus for dispensing a coating solution is disclosed in Japanese Patent Publication (Examined) No. 1989-52065 (Japanese Patent Publication (Unexamined) No. 1985-12175), for example.
This type of solution dispensing apparatus has a sealed container for storing the coating solution, which is pressurized by introducing nitrogen gas or helium gas thereinto. The coating solution is delivered under pressure from the container through supply piping, to drip onto a substrate surface.
A different type of apparatus employs a bellows pump for delivering a photoresist as a coating solution to drip onto a substrate surface. That is, the bellows pump sucks a coating solution such as a photoresist, and delivers the coating solution for application to a substrate surface.
The known apparatus have various drawbacks as set out hereunder.
The apparatus utilizing gas pressure encounters the following inconvenience when nitrogen gas is used for introduction into the sealed container.
Nitrogen gas is readily soluble in the coating solution. The nitrogen gas dissolved in the coating solution may emerge in bubbles during transmission of the coating solution or may cause degassing after application to a substrate. This results in pinholes appearing in the film formed on the substrate, or in uneven application of the solution, thereby lowering the yield of products.
Where helium gas is used instead of nitrogen gas, the gas is less soluble in the coating solution. However, helium gas is expensive and poses a different problem of high running cost.
The apparatus using a bellows pump and not a gas is free from the problems of gas dissolution in the coating solution and high running cost, but has the following drawback where the coating solution is the type that forms silica film.
The bellows pump sucks and delivers the coating solution with expansion and contraction of the bellows. However, by reason of the pump structure, the coating solution drawn in is not entirely forced out, with a possibility that part of the coating solution remains in the bellows over a long period of time. The coating solution for forming silica film, generally, becomes crystallized with ease. If the coating solution for forming silica film remains in the bellows, crystals of the coating solution will be formed in the bellows. As a result, the bellows pump will deliver the coating solution intermingled with crystals, for application to a substrate surface. When this coating solution forms a film on the surface of a spinning substrate, the crystals will adhere to the substrate surface. This results in uneven application of the solution around the crystals, thereby impairing uniformity of the film formed.